Conventionally, power plants are widely used in which a steam turbine generator is driven by steam and converts the steam into power. FIG. 29 is a diagram showing an ordinary thermal power plant. The thermal power plant comprises a boiler 10 which generates steam and a plurality of turbines 14, 16, and 18 which drives a generator 12 using steam from the boiler 10. Feed water is supplied to the boiler 10 from a feed pump 20 via a high-pressure feed water heater 22, and the boiler 10 generates main steam by heating the feed water.
The main steam is supplied to a high-pressure turbine 14 via a governor valve 24. Exhaust steam of the high-pressure turbine 14 is supplied to a reheater inside the boiler 10 as low-temperature reheat steam. High-temperature reheat steam that is reheated by the reheater is supplied to an intermediate-pressure turbine 16, and exhaust steam of the intermediate-pressure turbine 16 is supplied to a low-pressure turbine 18. Exhaust hot steam of the low-pressure turbine 18 is introduced into a condenser 26.
Condensate generated by cooling of exhaust hot steam in the condenser 26 is supplied from a condensate pump 28 to a deaerator 32 via a low-pressure feed water heater 30. Bleed steam of the intermediate-pressure turbine 16 is supplied to the deaerator 32, and oxygen contained in feed water is removed by heat of the bleed steam. Feed water discharged from the deaerator 32 is supplied to the boiler 10 via the feed pump 20 and the high-pressure feed water heater 22.
In this case, the deaerator 32 has a deaerator water storage tank which stores deaerated feed water, and a deaerator water level adjustment valve 34 is provided on a condensate supply line from the condenser 26 to the deaerator 32. An amount of the feed water stored in the deaerator water storage tank is kept constant by the deaerator water level adjustment valve 34. Therefore, during stable operation, an amount of the condensate supplied to the deaerator 32, an amount of the feed water supplied to the boiler 10, and an amount of the steam bled from the intermediate-pressure turbine 16 are maintained at a given balance in the deaerator 32.
In such a power plant, output control is performed in accordance with a requested load instruction from a power system. For example, Patent Document 1 (Japanese Patent Application Laid-open No. 2009-300038) discloses a configuration in which governor valve opening control, fuel flow rate control, or feed water flow rate control is performed based on a requested load signal to a boiler. In addition, when a frequency fluctuation in a power system or a power plant occurs, frequency control by a governor is performed. As described above, in a conventional power plant, output control in accordance with a requested load instruction or a frequency fluctuation is performed by means of steam flow rate control, steam pressure control, fuel flow rate control, air flow rate control, or governor valve opening control by a steam system of a boiler or the like.
Meanwhile, in recent years, in addition to large-scale power plants such as that described above, such output control is increasingly being introduced into power systems of dispersed power sources utilizing natural energy as exemplified by a wind farm or a large-scale photovoltaic power station. Since an amount of utilizable natural energy fluctuates due to reasons such as wind dying down in the case of wind power, output fluctuations occur with a dispersed power source. Since fine frequency fluctuations occur in the power system due to such output fluctuations, a power plant requires output control capable of stabilizing such frequency fluctuations. Conventionally, frequency control by means of opening control of a governor valve has been performed in a power plant in order to stabilize such frequency fluctuations.
Patent Document 1 Japanese Patent Application Laid-open No. 2009-300038
However, when frequency fluctuations of a power system are significant, conventional frequency control by means of opening control of a governor valve is, by itself, unable to sufficiently suppress the fluctuations and necessitates the fluctuations to be suppressed by control of a steam system of a boiler. However, with control by the steam system of a boiler such as that disclosed in Patent Document 1 or the like, for example, dead time or delay in response to a fuel flow rate instruction or combustion delay of coal as fuel in the boiler inhibits responsiveness of the control and prevents swift fluctuation suppression. In particular, since frequency fluctuations in a dispersed power source occur in relatively short time periods, it is extremely difficult to maintain a high precision in which the output control conforms to the frequency fluctuations.